The production of a redistribution layer, which realizes an electrical connection between active structures and an associated bonding pad on a wafer or a 3D structure in the form of a compliant element, is relatively complicated and requires a plurality of photolithographic process steps. Thus, the wafer first has to be coated with a photoresist, which subsequently has to be exposed and developed. This is followed by coating with a metal layer, after which the photoresist is stripped. These process steps have to be repeated until the desired layer sequence is achieved. These process steps are represented in principle in a scheme below.
Thus, in a method employed in practice at the present time, the necessary patterning of the gold layer is realized by means of a customary lithographic process. The redistribution layer is produced in this case in that, after the deposition of a seed layer and Cu/Ni layer situated thereon of the redistribution layer, the gold is deposited on the entire redistribution layer. The actual electrical conductor is in this case the Cu layer having the lowest electrical resistance.
This method can be represented in summary as follows:                a) Deposition of the seed layer        b) EPR1 (epoxy photoresist 1): Coating and patterning (lithography step 1)        c) Reroute plating, production of the Cu/Ni layer sequence on the seed layer        d) Coating of the reroute trace with Au        e) EPR2 (epoxy photoresist 2): Coating and patterning (lithography step 2)        f) (as required) selective etching of the Au layer (wet etching, CMP, or removal/stripping).        
In this case, the Ni layer serves as an adhesion layer for the Cu layer and the latter in turn serves as an adhesion layer for the Au covering layer. Since the Au layer itself cannot oxidize, it serves, on the one hand, as a secure adhesion layer for a solder material, in order for example to connect a 3D structure to a connection pad of a printed circuit board, which is usually composed of Cu, and, on the other hand, as a protective layer for the Cu layer situated underneath. In other words, the Cu layer is largely protected from corrosion by the Au layer.
A particular disadvantage in the case where the redistribution layer is constructed with such a structure is the fact that the side edges of the redistribution layer are not protected at all against corrosion and oxidation. This means that the possibly laterally penetrating corrosion or advancing oxidation may ultimately lead locally to a destruction of the redistribution layer, thereby limiting the service life of the electronic component provided with such a redistribution layer.
During front-end IC production, the Cu metallization is protected from oxidation during CMP (chemical mechanical polishing) by BTA (benzotriazole), and afterward by a liner (TiN) and an oxide.
In the production of printed circuit boards (PCBs), the industry uses BTA or another organic layer in order to provide the printed circuit board with protection from oxidation prior to soldering. Furthermore, the conductive tracks on a printed circuit board are comparatively thick and wide, with the result that corrosion problems are not especially significant in this case.
The situation is different, however, in the case of redistribution layers on wafers or chips. In this case, the problem has hitherto been solved by encapsulating the redistribution layers through galvanic or electrical coating. However, this requires an additional lithography step, which increases the process complexity and costs.
Another possibility for protecting the Cu layer consists in using the soldering process in order to enclose the Cu layer by means of a UBM (under bump metallization). In this case, the redistribution layer is completely encapsulated, although this is not suitable for long conductive tracks and, moreover, would lead to a significant increase in costs.